In recent years, with a conspicuous spread of a patch antenna for use in a cellular phone, a cordless phone, a RFID, and the like, and a lens antenna for use in a radio telescope, a millimeter-wave radar, and the like, and with an outstanding progress of a satellite communication apparatus, there is a growing demand for making the frequency of a communication signal high and communication apparatuses compact.
As the dielectric constant of a material of an antenna incorporated in the communication apparatus becomes higher, it is possible to make the communication signal higher and the communication apparatus compact. The dielectric constant is a parameter indicating the degree of a polarization inside the dielectric. Therefore when the material of the antenna having a high dielectric constant can be used, it is possible to make the frequency of the communication signal high and hence shorten circuits and make the communication apparatus compact.
Conventionally, as a highly dielectric elastomer, the fibrous metal salt of titanic acid and/or the composite fiber of the metal salt of titanic acid and the amorphous titanium oxide integrally enclosing the fibrous metal salt of titanic acid therewith to form the integrated composite fiber are known. The integrated composite fiber having the mol ratio between metals M and Ti in the range of 1.005 to 1.5 is mixed with the elastomer at 5 to 80 wt % with respect to the total weight of the highly dielectric elastomer composition to allow the highly dielectric elastomer composition to have a high permittivity (patent document 1).
300 to 500 parts by weight of barium titanate powder having a dielectric constant not less than 2000 in the range of a room temperature to 90° C. is mixed with 100 parts by weight of ethylene propylene rubber or the like crosslinked with the peroxide to form the high-permittivity rubber composition having a dielectric constant set to not less than 10 and preferably not less than 20 (patent document 2).
But in the above-described highly dielectric elastomer composition of the example (patent document 1) composed of the elastomer and the composite fiber mixed therewith, it is difficult to select the kind of the elastomer and the composite fiber, and owing to the directionality of the fibrous filling material, the molding of the highly dielectric elastomer composition is anisotropic. Thereby the linear expansion and the dielectric characteristic become unstable. Thus it is difficult to obtain a material having a high permittivity and a low dielectric loss tangent.
The high-permittivity rubber composition (patent document 2) is an insulating material which is disposed to relax an electric field mismatching which is liable to be generated at a connection position of power cables, a termination portion thereof, and the like. As the property of the barium titanate powder, it has a large dielectric loss tangent. Thus the barium titanate powder is unsuitable for an electronic part such the material of an antenna.
As the use mode of the communication apparatus becomes diversified, the communication apparatus is demanded to have little variations in the electrical characteristic in the range from a low temperature to a high temperature. When the conventional highly dielectric elastomer composition is used for an electronic part that is used in a wide temperature region, the electrical characteristic of the electronic part changes greatly.
Further, to comply with the demand of miniaturization of the communication apparatus, an antenna body is formed by molding a dielectric resin material which has a small specific gravity and a small dielectric loss and is advantageous for obtaining a high gain, and an electrode is formed on the obtained molding to form a dielectric antenna.
As a conventional dielectric antenna having an electrode plated with a metal, the rubber-like elastic material soluble in the inorganic filler and in the solvent is mixed with the styrene polymer having the syndiotactic structure (SPS) to obtain the composite dielectric material, the surface of which is roughened by etching treatment to plate the surface at an improved degree. The composite dielectric material is used as an antenna (patent document 3). A resin difficult to be plated and a resin easy to be plated are used in combination. The resin easy to be plated is used on the surface on which the electrode is formed so that it is used as the material of the antenna (patent document 4). Further the electrode is made of the copper foil pattern (patent document 5). In addition, the electrode is formed on the antenna body made of ceramics by screen printing (patent document 6).
When a resin material is used as a dielectric substrate of an antenna, it is necessary to form an electrode. As methods of forming the electrode, a metal plating method, a metal foil-bonding method, and screen printing method, and the like are known. Metal plating treatment is difficult to be made. Further the metal plating treatment has a problem that a specific foundation treatment is required, as disclosed in the patent documents 3 and 4. Further when the electrode is formed by plating treatment, the electrode has a low degree of adhesion to the material of the antenna to a low degree after the foundation treatment is made. Thus there is a fear that the dielectric characteristic deteriorates. Thus the metal plating processing is unfavorable. When the copper foil is used as the electrode, as disclosed in the patent document 5, the electrode is liable to be oxidized. Thus when a working temperature rises, the electrode is oxidized and the permittivity decreases. The screen printing is applied to form the electrode on the antenna body made of ceramics but not applied to the antenna body made of the elastomer. Screen printing paste used to form the electrode of the antenna made of ceramics contains silica. A product including the printed paste is calcined at a high temperature of 500 to 600° C. to vitrify it so that Ag powder is fixed. Because the elastomer is decomposed at a high temperature of 500° C., the screen printing method cannot be used for the antenna composed of the elastomer.
Patent document 1: Japanese Patent Application Laid-Open No. 09-031244
Patent document 2: Japanese Patent Application Laid-Open No. 2003-138067
Patent document 3: Japanese Patent Application Laid-Open No. 2001-143531
Patent document 4: Japanese Patent Application Laid-Open No. 2003-078322
Patent document 5: Japanese Patent Application Laid-Open No. 07-066620
Patent document 6: Japanese Patent Application Laid-Open No. 06-029727